1. Field of the Invention
The present invention relates to a method of and an apparatus for navigation display to display particular points in a graphic element, and more particularly, to a method of and an apparatus for navigation display to display guidance positional information about a position which is to be possibly indicated next, when a graphic element such as a straight line is to be created in connection to or contact with an existing graphic element such as a line segment, a circle, or an arc in an interactive system such as a computer-aided design (CAD) system.
2. Description of the Related Art
Two-dimensional CAD systems are capable of generating or editing drawings such as design drawings on a graphic display screen using basic graphic elements including a straight line, a circle, an arc, etc. When a drawing is completed on the graphic display screen, it may be either outputted as a hardcopy drawing by a plotter or stored in a storage medium for subsequent use.
To create a circle, for example, by a two-dimensional CAD system, the circle can easily be created on a graphic display screen based on commands entered from a keyboard or input signals from a digitizer. However, a mouse allows the user to carry out such an operation that a circle is created or, otherwise edited by moving a created circle more intuitively on the graphic display screen.
To create a straight line segment, for example, on a graphic display screen, a mouse cursor on the graphic display screen is moved to a position where one end of the line segment is to be located, and the position is clicked by a button on the mouse so as to serve as a start of the line segment. Then, a position on the graphic display screen where the other end of the line segment is to be located is clicked by the mouse, and serves as an end of the line segment. The end points of the line segment are indicated in this manner. For determining the end points of the line segment, therefore, it is necessary to move the mouse cursor to the positions which are to serve as the end points and then to hit or click the positions with the mouse.
When such a line segment is to be created in relation to a graphic element, e.g., to be connected to a generated graphic element, however, the user is often unable to easily recognize which position on the graphic element is to be used as an end point of the line segment that is to be created. Frequently, a drawing may be created which is different from what the user has had in mind. In such a case, efforts to create a drawing are repeated several times until finally a drawing will be produced as desired by the user. Such a process is however wasteful. Furthermore, if a drawing contains an intricate area, then it is highly difficult to find a position to be hit or clicked in such an intricate area. Accordingly, a process of generating a drawing which contains an intricate area needs an extra operation to enlarge the intricate area.
Among the conventional CAD systems is a CAD system with a navigation display function to display a particular point closest to a cursor on a graphic display screen when the cursor approaches a graphic element which has been generated. The navigation display function helps the user recognize the displayed particular point as one candidate for a position to be hit or clicked. The CAD system with the navigation display function can easily be operated by the user.
However, the conventional navigation display function displays only one of the particular points that are possessed by the generated graphic element, which one of the particular points which is closest to the cursor. Consequently, the user is given only partial information about the particular points of the graphic element, and such partial information tends to invite erroneous operations due to insufficient navigation display.